In the document Mishra, N.; Chebrolu, K.; Raman, B.; Pathak, A., “Wake-on-WLAN”, in Proceedings of WWW Conference 2006, authors propose using “sleeping” WLAN stations featuring a low-power sensor to monitor the stations' working channel. Upon energy detection in such channel, the sensor is in charge of waking up the main WLAN transceiver. Although this approach has the advantage of not requiring a custom transmitter, the number of false positives detected by the sensor is too high, even when the system operates in a lightly loaded ISM (Industrial, Science & Medical) channel.
The same authors tried to overcome this issue by incorporating address patterns in the incoming activation signals, as explained in the document Mishra, N.; Golcha, D.; Bhadauria, A.; Raman, B.; Chebrolu, K., “S-WOW: signature based Wake-on-WLAN”, in Proceedings of COMSWARE 2007. In this second proposal, they use alternate long standard Wi-Fi frames and silence periods to code address patterns in a Non-Return-to-Zero (NRZ) manner. Unfortunately, for communications with purposes other than Wake-up, the data-rate of this strategy is too low (the wake-up signal can last up to 1 second). In addition, due to the long silence periods (˜20 ms), this system is still prone to interferences.
Similar to the previous approach, the authors Kondo, Y.; Yomo, H.; Tang, S.; Iwai, M.; Tanaka, T.; Tsutsui, H.; Obana, S., in their article “Energy-efficient WLAN with on-demand AP Wake-up using IEEE 802.11 frame length modulation,” Computer Communications, Vol. 35, No. 14, p. 1725-1735, August 2012, code unique addresses by varying the IEEE 802.11 frame lengths. However, although more resilient to interferences, this approach is still slow, and thus inappropriate for long data transmissions. Furthermore, the current consumption of its associated Wake-up Receiver design is in the milliAmper (mA) order. This is simply unacceptable for Wake-up Radio systems, where receivers should not employ more than 10 μW.
A strategy consisting in varying the size of IEEE 802.11 frames (or another wireless technology) is also proposed in the U.S. Pat. No. 8,259,691B2, where the authors use such procedure to remotely configure a low-power device which provides no other human interaction interface (i.e. no keyboard, touchscreen, etc.). Data is decoded by comparing the relative length difference between two consecutive frames in such a way that a change in the frame length represents a binary ‘0’. Analogously, no change in the frame length represents a ‘1’. Again, surrounding transmissions, even from a different standards such as IEEE 802.15.4, may invalidate an ongoing Wake-up procedure.
In application US20120120859 A1, the utilization of unused OFDM subcarriers at the edges of the IEEE 802.11a/g spectrum is proposed to embed Wake-up signals. According to this standard, there are 52 OFDM subcarriers, but the authors propose to use up to 4 additional subcarriers. This approach requires severe hardware and software modifications in standard-compliant devices. Furthermore, it is limited to IEEE 802.11 transmissions based on OFDM, which allow for shorter communication range than the ones based on DSSS (Direct Sequence Spread Spectrum).
In the application US20060063484 A1, an Amplitude Modulated (AM) signal is employed to transmit data at low bit-rates. This signal is generated by using IEEE 802.11 frames where a standard preamble is followed by a non-standard amplitude-modulated waveform, generated by a gain control mechanism installed in the transmitter. Note that this approach also requires modifying the hardware of the IEEE 802.11 devices.
To summarize, the vast majority of works in the related literature are focused on the implementation of Wake-up signals, following different strategies. However, such implementations do not consider or implement a low frequency, low-rate signal using standard-compliant IEEE 802.11/802.15 frames. In fact, even if some of the previous solutions could be used for similar purposes, they would be either inefficient or require intensive hardware modifications. These two issues strongly limit their potential reproducibility and applicability. In comparison, the invention proposed in the current document does not require any hardware modification on IEEE 802.11 or IEEE 802.15 commercial devices; it can be used to generate a Wake-up message (or any other low-rate data) by emulating a low frequency signal by means of only standard IEEE 802.11 or IEEE 802.15 frames.